The present invention relates to an electrostatographic apparatus such as an electrostatic copying machine comprising means for automatically determining whether an original document being reproduced is a printed document or a photographic document and controlling at least one operating parameter of the apparatus such as a developing bias voltage in accordance with the determination.
A fully automatic electrostatic copying machine which is capable of producing satisfactory copies of both printed documents and photographic documents has heretofore not been developed. A printed document is considered to be a document which bears only alphanumeric or other characters, symbols, etc. such as a page of a book. A photographic document is considered to be a document which bears a pictorial scene having various shades of grey in addition to black and white.
Copying machines using the edge effect are able to produce good copies of printed documents, but fail when used for photographic documents since the large dark image areas are lost. Copying machines using a magnetic brush developing unit and automatic bias control produce generally satisfactory copies of photographic documents but poor copies of printed documents since the white background areas appear grey.
It is possible to provide a copying machine having a magnetic brush developing unit with a changeover switch which the operator sets in accordance with the type of documents being copied. The switch controls the developing bias voltage, document illumination intensity, etc. in dependence on the type of document. However, such an arrangement is not entirely satisfactory since it is desired to provide a completely automatic copying machine which does not comprise any manual settings or adjustments.